Incremental position encoders employing electrooptics are known in the prior art. One specific example of an optical encoder providing for phase adjustment via adjustment of focus of light beams from a light source is described in the Trump U.S. Pat. No. 3,400,275. Another specific example of a modular shaft encoder including on-board electronic circuitry and providing phase shifted outputs in quadrature is the referenced Freyer et al. U.S. Pat. No. 4,184,071. One of the advantages claimed for that approach was that of modularity which enabled the encoder assembly to be pre-calibrated prior to its installation on a motor shaft. Difficulties otherwise associated with installation and trim were said to be minimized.
A further specific example is found in the referenced Harrison et al. U.S. Pat. No. 4,396,969, particularly as found in the discussion of FIGS. 5-7 thereof. In the Harrison et al. patent, the optical encoder was provided as a key element of a servo system for a rotating, non-removable disk data storage device. The drive included a novel rotary actuator, U.S. Pat. No. 4,490,635, which had a rotary displacement of only about 30.degree.. The encoder thereof included a scale edge mounted to the rotary actuator and an encoder housing mounted to the base casting of the drive. The photodetector array put out analog phase signals in quadrature which were selected to control an analog servo loop having an offset value derived from a data storage surface in track following mode. The quadrature signals additionally provided indications of track boundary crossings and were used by a programmed microprocessor to determine transducer relative position during track seeking operation. The other referenced applications comprise further improvements in the encoder described in the referenced Harrison et al. patent.
One of the drawbacks of the prior optical encoder described in the Harrison et al. patents and e.g. the Nixon U.S. Pat. No. 4,625,109 was that it was not a compact modular assembly which could be assembled, electrically tested and calibrated with its associated electronics as a unit prior to its installation into the disk drive.
Another drawback of the referenced prior optical encoder was that the optical path between the infrared LED light source and the photodetector was so short that light beam collimation was minimal leading to an undesirable penumbra effect. Poorly collimated light from the light source causes the amplitude of the encoder signal to degrade very rapidly as a function of scale to reticle gap; i.e., it forces the encoder to be operated at a smaller gap and makes it inordinately sensitive to changes in gap length.
A further drawback of the referenced prior optical encoder was that its manner of mounting did not facilitate ready alignment and smooth trim during the assembly and calibration process.